...salt are formed by combination of strong acid and weak base .

Solution:
A salt of strong acid and weak base forms a acidic salt (pH<7)
for example HCl (strong acid) + NH₄OH (weak base) $\stackrel{}{\to }$ NH₄Cl (acidic salt) + H₂O
                    H₂SO₄ (strong acid) + LiOH (weak base) $\stackrel{}{\to }$Li₂SO₄ (acidic salt) + 2H₂O